Mesozoic Earth History

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Title: Mesozoic Earth History

1
Chapter 14
Mesozoic Earth History
2
Nevadan Orogeny and Gold

Approximately 150 to 210 million years after
the emplacement of massive plutons created the
Sierra Nevada
Nevadan orogeny
gold was discovered at Sutter's Mill
on the South Fork of the American River at
Coloma, California
On January 24, 1848, James Marshall,
a carpenter building a sawmill for John Sutter,
found bits of the glittering metal in the mill's
tailrace

3
Gold Rush

Soon, settlements throughout the state
were completely abandoned as word
of the chance for instant riches
spread throughout California
Within a year after
the news of the gold discovery reached the East
Coast,
the Sutter's Mill area was swarming with more
than 80,000 prospectors,
all hoping to make their fortune

4
Gold Mining

By 1852,
mining operations were well underway
on the American River near Sacramento

5
Prospecting Was Very Hard Work

At least 250,000 gold seekers
prospected the Sutter's Mill area,
and although most were Americans,
they came from all over the world,
even as far away as China
Most of them thought
the gold was simply waiting to be taken,
and didn't realize that prospecting
was very hard work

6
Shopowners Made More Money

Of course, no one gave any thought
to the consequences of so many people converging
on the Sutter's Mill area,
all intent on making easy money
In fact, life in the mining camps
was extremely hard and expensive
Frequently, the shop owners and traders
made more money than the prospectors

7
Abandoning Their Dream

In reality, only a small percentage of
prospectors
ever hit it big
or were even moderately successful
The rest barely eked out a living
until they eventually abandoned their dream and
went home

8
Placer Gold

The gold these prospectors sought was mostly in
the form of placer deposits
Placer deposits form
when gold-bearing igneous rocks weather
and stream transport mechanically separates
minerals
by density
Although many prospectors searched for the mother
lode,
all of the gold recovered during the gold rush
came from placers

9
Gold Panning

Panning is a common method of mining placer
deposits
In this method,
a shallow pan is dipped into a streambed,
the material is swirled around
and the lighter material is poured off
Gold, being about six times heavier
than most sand grains and rock chips,
concentrates on the bottom of the pan
and can then be picked out

10
200 million in gold

Although some prospectors
dug 30,000 worth of gold dust a week
out of a single claim
and some gold was found sitting on the surface
most of this easy gold was recovered
very early during the gold rush
Most prospectors made only a living wage working
their claims
Nevertheless, during the five years
from 1848 to 1853
that constituted the gold rush proper,
miners extracted more than 200 million in gold

11
Mesozoic Era

The Mesozoic Era
251 to 66 million years ago
was an important time in Earth history
The major geologic event
was the breakup of Pangaea,
which affected oceanic and climatic circulation
patterns
and influenced the evolution of the terrestrial
and marine biotas

12
Other Mesozoic Events

Other important Mesozoic geologic events
resulting from plate movement
include
the origin of the Atlantic Ocean basin
and the Rocky Mountains
accumulation of vast salt deposits
that eventually formed salt domes
adjacent to which oil and natural gas were
trapped
and the emplacement of huge batholiths
accounting for the origin of various mineral
resources

13
The Breakup of Pangaea

Just as the formation of Pangaea
influenced geologic and biologic events
during the Paleozoic,
the breakup of this supercontinent
profoundly affected geologic and biologic events
during the Mesozoic
The movement of continents
affected the global climatic and oceanic regimes
as well as the climates of the individual
continents

14
Effect of the Breakup

Populations became isolated
or were brought into contact
with other populations,
leading to evolutionary changes in the biota
So great was the effect of this breakup
on the world,
that it forms the central theme of the Mesozoic

15
Progress of the Breakup

The breakup of Pangaea
began with rifting
between Laurasia and Gondwana during the Triassic
By the end of the Triassic,
the expanding Atlantic Ocean
separated North America from Africa
This change was followed
by the rifting of North America from South
America
sometime during the Late Triassic and Early
Jurassic

16
Paleogeography of the World

During the Triassic Period

17
Paleogeography of the World

During the Jurassic Period

18
Paleogeography of the World

During the Late Cretaceous Period

19
Oceans Responded to Continental Separation

Separation of the continents
allowed water from the Tethys Sea
to flow into the expanding central Atlantic
Ocean,
while Pacific Ocean waters
flowed into the newly formed Gulf of Mexico,
which at that time was little more than a
restricted bay
Evaporites formed in these areas

20
Early Mesozoic Evaporites

Evaporites accumulated in shallow basins
as Pangaea broke apart during the Early Mesozoic
Water from the Tethys Sea flowed into the Central
Atlantic Ocean

21
Early Mesozoic Evaporites

Water from the Pacific Ocean flowed into the the
newly formed Gulf of Mexico

Marine water from the south flowed into the area
that would eventually become the southern
Atlantic Ocean

22
Evaporite Deposits

During that time, these areas were located
in the low tropical latitudes
where high temperatures
and high rates of evaporation
were ideal for the formation
of thick evaporite deposits

23
Further Breakup

During the Late Triassic and Jurassic periods,
Antarctica and Australia,
which remained sutured together,
began separating from South America and Africa
Also during this time,
India began rifting from the Gondwana continent
During the Jurassic,
South America and Africa began separating

24
Paleogeography of the World

During the Jurassic Period

25
Thick Evaporites from the Southern Ocean

The subsequent separation of South America and
Africa
formed a narrow basin
where thick evaporite deposits
accumulated from the evaporation
of southern ocean waters

26
Thick Southern Ocean Evaporites

Marine water flowed into the southern Atlantic
Ocean from the south

27
Tethys Sea

During this time, the eastern end of the Tethys
Sea
began closing
as a result of the clockwise rotation
of Laurasia and the northward movement of Africa
This narrow Late Jurassic and Cretaceous seaway
between Africa and Europe
was the forerunner
of the present Mediterranean Sea

28
End of the Cretaceous

By the end of the Cretaceous,
Australia and Antarctica had separated,
India was nearly to the equator,
South America and Africa were widely separated,
and Greenland was essentially an independent
landmass

29
Paleogeography of the World

During the Late Cretaceous Period

30
Higher Heat Flow Caused Sea Level Rise

A global rise in sea level
during the Cretaceous
resulted in worldwide transgressions
onto the continents
These transgressions were caused
by higher heat flow along the oceanic ridges
caused by increased rifting
and the consequent expansion of oceanic crust

31
Middle Cretaceous Sea Level Was High

By the Middle Cretaceous,
sea level was probably as high
as at any time since the Ordovician,
and approximately one-third of the present land
area
was inundated by epeiric seas

32
Paleogeography of the World

During the Late Cretaceous Period

33
Final Stage in Pangaea's Breakup

The final stage in Pangaea's breakup
occurred during the Cenozoic
During this time,
Australia continued moving northward,
and Greenland completely separated
from Europe and North America
and formed a separate landmass

34
The Effects on Global Climates and Ocean
Circulation Patterns

By the end of the Permian Period,
Pangaea extended from pole to pole,
covered about one-fourth of Earth's surface,
and was surrounded by Panthalassa,
a global ocean that encompassed about 300 degrees
of longitude
Such a configuration exerted tremendous influence
on the world's climate
and resulted in generally arid conditions
over large parts of Pangaea's interior

35
Paleogeography of the World

For the Late Permian Period

36
Ocean Currents and Continents

The world's climates result from the complex
interaction between
wind and ocean currents
and the location and topography of the continents
In general, dry climates occur
on large landmasses
in areas remote from sources of moisture
and where barriers to moist air exist,
such as mountain ranges
Wet climates occur
near large bodies of water
or where winds can carry moist air over land

37
Climate-Sensitive Deposits

Past climatic conditions can be inferred from
the distribution of climate-sensitive deposits
Evaporites are deposited
where evaporation exceeds precipitation
While dunes and red beds
may form locally in humid regions,
they are characteristic of arid regions
Coal forms in both warm and cool humid climates
Vegetation that is eventually converted into coal
requires at least a good seasonal water supply
Thus, coal deposits are indicative of humid
conditions

38
Evaporites, Red Beds, Dunes, Coal

Widespread Triassic evaporites, red beds, and
desert dunes
in the low and middle latitudes
of North and South America, Europe, and Africa
indicate dry climates in those regions,
while coal deposits
are found mainly in the high latitudes,
indicating humid conditions
These high-latitude coals are analogous to
today's Scottish peat bog
or Canadian muskeg

39
Bordering the Tethys Sea

The lands bordering the Tethys Sea
were probably dominated by seasonal monsoon rains
resulting from the warm, moist winds
and warm oceanic currents
impinging against the east-facing coast of
Pangaea

40
Faster Circulation

The temperature gradient
between the tropics and the poles
also affects oceanic and atmospheric circulation
The greater the temperature difference
between the tropics and the poles,
the steeper the temperature gradient
and the faster the circulation of the oceans and
atmosphere

41
Global Temperature Gradient

The breakup of Pangaea
during the Late Triassic
caused the global temperature gradient to
increase
because the Northern Hemisphere continents
moved farther northward,
displacing higher-latitude ocean waters
Decrease in temperature in the high latitudes
and the changing positions of the continents,
caused the steeper global temperature gradient
Thus, oceanic and atmospheric circulation
patterns
greatly accelerated during the Mesozoic

42
Oceanic Circulation Evolved

From a simple pattern in a single ocean
(Panthalassa) with a single continent (Pangaea)

43
Oceanic Circulation Evolved

to a more complex pattern in the newly formed
oceans of the Cretaceous Period

44
Areas Dominated by Seas are Warmer

Oceans absorb about 90 of the solar radiation
they receive,
while continents absorb only about 50,
even less if they are snow covered
The rest of the solar radiation is reflected back
into space
Therefore, areas dominated by seas are warmer
than those dominated by continents

45
Oceans Still Quite Warm

By knowing the distribution of continents and
ocean basins,
geologists can generally estimate
the average annual temperature
for any region on Earth,
as well as determining a temperature gradient
Though the temperature gradient and seasonality
on land
were increasing during the Jurassic and
Cretaceous,
the middle- and higher-latitude oceans
were still quite warm

46
Equable Worldwide Climate

Higher-latitude oceans remained warm
because warm waters from the Tethys Sea
were circulating to the higher latitudes
The result was a relatively equable worldwide
climate
through the end of the Cretaceous

47
The Mesozoic History of North America

In North America, the beginning of the Mesozoic
Era
was essentially the same in terms of tectonism
and sedimentation
as the preceding Permian Period
Terrestrial sedimentation continued over much of
the craton,
while block faulting and igneous activity
began in the Appalachian region
as North America and Africa began separating

48
Permian Period

Paleogeography of North America during the
Permian Period

49
Triassic Period

Paleogeography of North America during the
Triassic Period

50
Gulf of Mexico

The newly forming Gulf of Mexico
experienced extensive evaporite deposition
during the Late Triassic and Jurassic
as North America separated from South America

51
Jurassic Period

Paleogeography of North America during the
Jurassic Period

52
Global Sea-Level Rise

A global rise in sea level
during the Cretaceous
resulted in worldwide transgressions
onto the continents such that marine deposition
was continuous over much of the North American
Cordillera

53
Volcanic Island Arc at the Western Edge of the
Craton

A volcanic island arc system
that formed off the western edge of the craton
during the Permian
was sutured to North America
sometime later during the Permian or Triassic
This event is referred to as the Sonoma orogeny

54
Cordilleran Area

During the Jurassic,
the entire Cordilleran area
was involved in a series
of major mountain-building episodes
that result in the formation of the Sierra
Nevada,
the Rocky Mountains,
and other lesser mountain ranges
Although each orogenic episode
has its own name,
the entire mountain-building event
is simply called the Cordilleran orogeny

55
Next, Specific Regions

Keeping in mind this simplified overview
of the Mesozoic history of North America,
we will now examine the specific regions of the
continent

56
Continental Interior

Recall that the history of the North American
craton
can be divided into unconformity-bound sequences
reflecting advances and retreats of epeiric seas
over the craton
While these transgressions and regressions
played a major role in the Paleozoic geologic
history of the continent,
they were not as important during the Mesozoic

57
Cratonic Sequences of North America

White areas represent sequences of rocks

that are separated by large-scale uncon-formities
shown in brown

58
Continental Interior With Inundation

Cratonic sequences are less important because
most of the continental interior
during the Mesozoic
was well above sea level
and did not experience epeiric sea inundation
As we examine the Mesozoic history
of the continental margin regions of North
America
we will combine the two cratonic sequences,
the Absaroka Sequence
Late Mississippian to Early Jurassic
and Zuni Sequence
Early Jurassic to Early Paleocene

59
Cratonic Sequences of North America

Absaroka sequence

Zuni sequence

60
Eastern Coastal Region

During the Early and Middle Triassic,
coarse detrital sediments derived from the
erosion of the recently uplifted Appalachians
Alleghenian Orogeny
filled the various intermontane basins
and spread over the surrounding areas
As erosion continued during the Mesozoic,
this once lofty mountain system was reduced to a
low-lying plain

61
Fault-block Basins

During the Late Triassic,
the first stage in the breakup of Pangaea began
with North America separating from Africa
Fault-block basins developed
in response to upwelling magma
beneath Pangaea
in a zone stretching
from present-day Nova Scotia to North Carolina

62
Triassic Fault Basins

Areas where Triassic fault-block basin deposits
crop out in eastern North America

63
Fault-Block Basins

After the Appalachians were eroded to a low-lying
plain
by the Middle Triassic,
fault-block basins formed
as a result of Late Triassic rifting
between North America and Africa

64
Newark Group

Erosion of the adjacent fault-block mountains
filled these basins with great quantities
up to 6000 m
of poorly sorted red nonmarine detrital sediments
known as the Newark Group

65
Down-dropped valleys accumulated sediments

Down-dropped valleys accumulated tremendous
thickness of sediments
and were themselves broken
by a complex of normal faults during rifting

66
Reptile Footprints

Reptiles roamed along the margins
of the various lakes and streams
that formed in these basins,
leaving their footprints and trackways
in the soft sediments
Although the Newark Group rocks contain numerous
dinosaur footprints,
they are almost completely devoid of dinosaur
bones!
The Newark Group is mostly Late Triassic,
but in some areas deposition began in the Early
Jurassic

67
Reptile Tracks

Reptile tracks in the Triassic Newark Group
were uncovered during the excavation
for a new state building in Hartford, Connecticut
Because the tracks were so spectacular,
the building side was moved
and the excavation was designated as a state park

68
Reptile Tracks
69
Igneous Activity

Concurrent with sedimentation
in the fault-block basins
were extensive lava flows
that blanketed the basin floors
as well as intrusions of numerous dikes and sills
The most famous intrusion
is the prominent Palisades sill
along the Hudson River
in the New York-New Jersey area

70
Palisades Sill of the Hudson River

This sill was one of many that were intruded into
the Newark sediments
during the Late Triassic rifting
that marked the separation
of North America from Africa

71
Passive Continental Margin

As the Atlantic Ocean grew,
rifting ceased along the eastern margin
of North America,
and this once active plate margin
became a passive, trailing continental margin
The fault-block mountains
that were produced by this rifting
continued eroding
during the Jurassic and Early Cretaceous
until all that was left was a large low-relief
area

72
Eastern Continental Shelf

The sediments produced
by this erosion
contributed to the growing eastern continental
shelf
During the Cretaceous Period,
the Appalachian region was re-elevated
and once again shed sediments
onto the continental shelf,
forming a gently dipping,
seaward-thickening wedge of rocks
up to 3000 m thick

73
Seaward-Thickening Wedge

The seaward-thickening wedge of rocks
is currently exposed
in a belt extending from
Long Island, New York,
to Georgia

74
Gulf Coastal Region

Paleogeographic Map of North America during the
Triassic Period
The Gulf Coastal region was above sea level until
the Late Triassic
-

75
Evaporites in Gulf of Mexico

As North America separated from South America
during the Late Triassic and Early Jurassic,
the Gulf of Mexico began to form
With oceanic waters flowing into
this newly formed, shallow, restricted basin,
conditions were ideal for evaporite formation
These Jurassic evaporites
are thought to be the source
for the Paleogene salt domes
found today in the Gulf of Mexico and southern
Louisiana

76
Jurassic Period

Paleogeographic reconstruction for the Jurassic
Period
The Gulf of Mexico began to form
with the precipitation of evaporites

77
Paleogene Salt Domes
78
Evaporite Deposition Ended

The history of the Paleogene salt domes
and their associated petroleum accumulations
will be discussed with the Cenozoic
By the Late Jurassic,
circulation in the Gulf of Mexico
was less restricted,
and evaporite deposition ended

79
Normal Marine Conditions

Normal marine conditions
returned to the area
with alternating transgressing and regressing
seas
The resulting sediments were
covered and buried by thousands of meters
of Cretaceous and Cenozoic sediments
During the Cretaceous,
the Gulf Coastal region,
like the rest of the continental margin,
was flooded by northward-transgressing seas

80
Cretaceous Period

Paleogeography of North America during the
Cretaceous Period
with its northward-transgressing seas

81
Transgressions and Regression

As a result of the transgression,
nearshore sandstones
are overlain by finer sediments
characteristic of deeper waters
Following an extensive regression
at the end of the Early Cretaceous,
a major transgression began
during which a wide seaway extended
from the Arctic Ocean to the Gulf of Mexico
Sediments that were deposited in the Gulf Coastal
region
formed a seaward-thickening wedge

82
Cretaceous Period

Paleogeography of North America during the
Cretaceous Period
Cretaceous Interior Seaway

83
Cretaceous Bivalve Reefs

Reefs were also widespread
in the Gulf Coastal region during the Cretaceous
Bivalves called rudists
were the main constituent
of many of these reefs
Because of their high porosity and permeability,
rudistoid reefs make excellent petroleum
reservoirs
A good example of a Cretaceous reef complex
occurs in Texas

84
Reef-Building Bivalves

Two genera of Cretaceous bivalves known as
rudists replaced corals as the main reef-building
animals of the Mesozoic

85
Gulf Shelf-Margin Facies

Early Cretaceous shelf-margin facies around the
Gulf of Mexico Basin
The reef trend shows as a black line

86
Reef Environments

Depositional environment and facies changes
across the Stuart City reef trend, South Texas

87
Rudist Reef Facies Patterns

Here the reef trend
had a strong influence
on the carbonate platform deposition of the
region
The facies patterns of these carbonate rocks
are as complex as those found
in the major barrier-reef systems
of the Paleozoic Era

88
Western RegionMesozoic Tectonics

The Mesozoic geologic history
of the North American Cordilleran mobile belt
is very complex,
involving the eastward subduction
of the oceanic Farallon plate
under the continental North American plate
Activity along this oceanic-continental
convergent plate boundary,
resulted in an eastward movement of deformation

89
Cordilleran Orogenic Activity

This orogenic activity
progressively affected
the trench and continental slope, the continental
shelf, and the cratonic margin,
causing a thickening of the continental crust
The accretion of terranes and microplates
played a significant role in this area

90
Sonoma Orogeny

Except for the Late Devonian-Early Mississippian
Antler orogeny,
the Cordilleran region of North America
experienced little tectonism during the Paleozoic
During the Permian, however, an island arc and
ocean basin formed
off the western North American craton
followed by subduction of an oceanic plate
beneath the island arc
and the thrusting of oceanic and island arc rocks
eastward against the craton margin

91
Sonoma Orogeny

This event, known as the Sonoma orogeny,
occurred at or near the Permian-Triassic boundary
and resulted in the suturing of island-arc
terranes
along the western edge of North America.

92
Triassic Period

Paleogeography of North America during the
Triassic Period
with a volcanic island arc in the west

93
Sonoma Orogeny

Tectonic activity that culminated
in the Permian-Triassic Sonoma orogeny

in western North America
was the result of a collision
between the southwestern margin of North America
and an island arc system

94
Oceanic-Continental Convergent Plate Boundary

Following the Late Paleozoic-Early Mesozoic
destruction of the volcanic island arc
during the Sonoma orogeny,
the western margin of North America
became an oceanic-continental convergent plate
boundary

95
Steeply Dipping Subduction Zone

During the Late Triassic,
a steeply dipping subduction zone developed
along the western margin of North America
in response to the westward movement
of North America over the Farallon plate
This newly created oceanic-continental plate
boundary
controlled Cordilleran tectonics
for the rest of the Mesozoic Era
and for most of the Cenozoic Era
This subduction zone marks the beginning
of the modern circum-Pacific orogenic system

96
Steeply Dipping Subduction Zone

Interpretation of the tectonic setting
of western North America
during the Late Triassic to Early Jurassic
Plutons of the Sierra Nevada began forming

97
Two Subduction Zones

Two subduction zones,
dipping in opposite directions from each other,
formed off the west coast of North America
during the Middle and early Late Jurassic

98
The North American Overrode the Farallon Plate

The more westerly subduction zone
was eliminated
by the westward-moving North American plate,
which overrode the oceanic Farallon plate

99
Continued Tectonic Evolution
100
Franciscan Complex

The Franciscan Complex, California,
which is up to 7000 m thick,
is an unusual rock unit
consisting of a chaotic mixture of rocks
that accumulated during the Late Jurassic and
Cretaceous
The various rock types include
graywacke, volcanic breccia, siltstone, black
shale,
chert, pillow basalt, and blueschist metamorphic
rocks
a low temperature, high pressure metamorphic rock

101
Franciscan Complex

The rock types suggest
that continental shelf, slope, and deep-sea
environments
were brought together
in a submarine trench
when North America overrode the subducting
Farallon plate

102
Franciscan Complex

Map showing the location of the Franciscan Complex

103
Depositional Environment

Reconstruction of the depositional environment
of the Franciscan Complex
during the Late Jurassic and Cretaceous periods

104
Franciscan Complex

Exposures of the Franciscan Complex along the
central California coast

105
Great Valley Group

East of the Franciscan Complex
and currently separated from it
by a major thrust fault
is the Great Valley Group
It consists of more than 16,000 m
of conglomerates, sandstones, siltstones, and
shales
These sediments were deposited
on the continental shelf and slope
at the same time the Franciscan deposits
were accumulating in the submarine trench

106
Great Valley Group Environment

Environments of the Great Valley Group
in relation to the Franciscan Complex

107
Cordilleran Orogeny

The general term Cordilleran orogeny
is applied to the mountain-building activity
that began during the Jurassic
and continued into the Cenozoic
The Cordilleran orogeny
consisted of a series
of individual mountain-building events
that occurred in different regions at different
times
Most of this Cordilleran orogenic activity
is related to the continued westward movement of
the North American plate

108
Cordilleran Mobile Belt

Mesozoic orogenies
occurring in the Cordilleran mobile belt

109
Nevadan Orogeny

The first phase of the Cordilleran orogeny,
the Nevadan orogeny,
began during the Late Jurassic
and continued into the Cretaceous
as large volumes of granitic magma
were generated at depth
beneath the western edge of North America
These granitic masses
ascended as huge batholiths
that are now recognized as
the Sierra Nevada, Southern California, Idaho,
and Coast Range batholiths

110
Cordilleran Mobile Belt

Mesozoic orogenies
occurring in the Cordilleran mobile belt

111
Batholiths

Location of Jurassic and Cretaceous batholiths
in western North America

112
Plutonic Activity Migrated Eastward

By the Late Cretaceous,
most of the volcanic and plutonic activity
had migrated eastward into Nevada and Idaho
This migration was probably caused
by a change from high-angle to low-angle
subduction,
resulting in the subducting oceanic plate
reaching its melting depth farther east

113
Eastward Migrating

A possible cause
for the eastward migration
of Cordilleran igneous activity
during the Cretaceous
was a change from high angle subduction to

114
Lower-Angle Subduction

to low-angle subduction
As the subducting plate
moved downward
at a lower angle,
its melting depth
moved farther to the east

115
Sevier Orogeny

Thrusting occurred progressively farther east
so that by the Late Cretaceous,
it extended all the way
to the Idaho-Washington border
The second phase of the Cordilleran orogeny,
the Sevier orogeny,
was mostly a Cretaceous event

116
Cordilleran Mobile Belt

Mesozoic orogenies
occurring in the Cordilleran mobile belt

117
Thrust Faults

Subduction of the Farallon plate
beneath the North American plate continued during
this time,
resulting in numerous overlapping,
low-angle thrust faults
in which blocks of older strata
were thrust eastward
on top of younger strata
This deformation produced
generally north-south-trending mountain ranges
that stretch from Montana to western Canada

118
Sevier Orogeny

Associated tectonic features

of the Late Cretaceous Sevier orogeny
caused by subduction of the Farallon plate
under the North American plate

119
Keystone Thrust Fault
120
Keystone Thrust Fault

The Keystone thrust fault is a major fault in the
Sevier overthrust belt
It is exposed west of Las Vagas, Nevada
The sharp boundary
between the light-colored Mesozoic rocks
and the overlying dark-colored Paleozoic rocks
marks the trace of the Keystone thrust fault

121
Keystone Thrust Fault
122
Laramide orogeny

During the Late Cretaceous to Early Cenozoic,
the final pulse of the Cordilleran orogeny
occurred
The Laramide orogeny
developed east of the Sevier orogenic belt
in the present-day Rocky Mountain areas
of New Mexico, Colorado, and Wyoming

123
Cordilleran Mobile Belt

Mesozoic orogenies
occurring in the Cordilleran mobile belt

124
Present-Day Rocky Mountains

Most of the features
of the present-day Rocky Mountains
resulted from the Cenozoic phase
of the Laramide orogeny

125
Mesozoic Sedimentation

Concurrent with the tectonism
in the Cordilleran mobile belt,
Early Triassic sedimentation
on the western continental shelf
consisted of shallow-water marine
sandstones, shales, and limestones
During the Middle and Late Triassic,
the western shallow seas
regressed farther west,
exposing large areas of former seafloor to erosion

126
Marine and Nonmarine Triassic Rocks

Marginal marine and nonmarine Triassic rocks,
particularly red beds,
contribute to the spectacular
and colorful scenery of the region
The Lower Triassic Moenkopi Formation
of the southwestern United States
consists of a succession of brick-red
and chocolate-colored mudstones

127
Triassic and Jurassic Formations

Triassic and Jurassic formations in the western
United States

128
Sedimentary Structures

Such sedimentary structures
as desiccation cracks and ripple marks,
as well as fossil amphibians and reptiles and
their tracks,
indicate deposition in a variety of continental
environments,
including stream channels, floodplains, and fresh
and brackish water ponds
Thin tongues of marine limestones
indicate brief incursions of the sea,
while local beds with gypsum and halite crystal
casts
attest to a rather arid climate

129
Shinarump and Chinle

Unconformably overlying the Moenkopi
is the Upper Triassic Shinarump Conglomerate,
a widespread unit generally less than 50 m thick
Above the Shinarump
are the multicolored shales, siltstones, and
sandstones
of the Upper Triassic Chinle Formation
This formation is widely exposed
throughout the Colorado Plateau
and is probably most famous for its petrified
wood,
spectacularly exposed in Petrified Forest
National Park, Arizona

130
Triassic and Jurassic Formations

Triassic and Jurassic formations in the western
United States

131
Petrified Wood and Plants Fossils

Whereas fossil ferns are found here,
the park is best known for
its abundant and beautifully preserved logs
of gymnosperms especially conifers
and plants called cycads
Fossilization resulted from the silicification of
the plant tissues
Weathering of volcanic ash beds
interbedded with fluvial and deltaic Chinle
sediments
provided most of the silica for silicification

132
Cycads
133
Fossilization

Some trees were preserved in place,
but most were transported during floods
and deposited on sandbars
and on floodplains,
where fossilization took place
After burial, silica-rich groundwater
percolated through the sediments
and silicified the wood

134
Other Fossils

Though best known for its petrified wood, the
Chinle Formation has also yielded fossils of
labyrinthodont amphibians,
phytosaurs,
and small dinosaurs

135
Upward in the Stratigraphy

The Wingate Sandstone,
a desert dune deposit,
and the Kayenta Formation,
a stream and lake deposit,
overlie the Chinle Formation
These two formations are well exposed
in southwestern Utah

136
Triassic and Jurassic Formations

Triassic and Jurassic formations in the western
United States

137
Early Jurassic Sandstones

The thickest and most prominent of the Jurassic
cross-bedded sandstones
is the Navajo Sandstone,
a widespread formation
that accumulated in a coastal dune environment
along the southwestern margin of the craton

138
Navajo Sandstone, Zion Canyon
139
Navajo Sandstone, Zion Canyon

View of East Entrance of Zion Canyon, Zion
National Park, Utah
The light-colored massive rocks
are the Jurassic Navajo Sandstone
while the slope-forming rocks below the Navajo
are the Upper Triassic Kayenta Formation

140
Navajo Sandstone, Zion Canyon
141
Navajo Sandstone's Large-Scale Cross-Beds

The Navajo Sandstone's most distinguishing
feature
is its large-scale cross-beds,
some of which are more than 25 m high

142
Navajo Sandstone

Large cross-beds of the Jurassic Navajo Sandstone
in Zion National Park, Utah

143
Sundance Sea

The upper part of the Navajo
contains smaller cross-beds
as well as dinosaur and crocodilian fossils
Marine conditions returned to the region
during the Middle Jurassic
when a seaway called the Sundance Sea
twice flooded the interior of western North
America

144
Sundance Sea

The resulting deposits,
the Sundance Formation,
were produced from erosion
of tectonic highlands to the west
that paralleled the shoreline

145
Sundance Sea Retreated Northward

These highlands
resulted from intrusive igneous activity
and associated volcanism
that began during the Triassic
During the Late Jurassic,
a mountain chain formed
in Nevada, Utah, and Idaho
as a result of the deformation
produced by the Nevadan orogeny
As the mountain chain grew
and shed sediments eastward,
the Sundance Sea began retreating northward

146
Morrison Formation

A large part of the area
formerly occupied by the Sundance Sea
was then covered
by multicolored sandstones, mudstones, shales,
and occasional lenses of conglomerates
that comprise the world-famous Morrison Formation
The Morrison Formation
contains the world's richest assemblage
of Jurassic dinosaur remains

147
Morrison Formation